The present invention concerns a balun, consisting of planar lines coupled to each other, in which one end of a first line running between a second and a third line serves as unbalanced signal input and the other end of this first line is contacted with ground and one end each of the other two lines forms a balanced signal input.
Baluns are known to represent transitions between balanced and unbalanced transmission lines. A balanced line exists when the signal transmitted over it does not have ground as reference potential. On the other hand, an unbalanced transmission line is in contact with ground on one side, so that a signal transmitted over it has ground as reference potential. Baluns of this type are used, for example, at the inputs and outputs of quadruplex mixers or amplifiers or modulators, etc. A balun, consisting of three coupled planar lines, is known, for example, from J. Villemazet, J. Dubouloy, M. Soulard, J. Cayrou, E. Husse, B. Cogo, J. Cazaus: New Compact Double Balanced Monolithic Down-Converter Application to a Single Chip MMIC Receiver for Satellite Equipment, IEEE MTT-S Digest, 1998, pages 853-856. An unbalanced gate is situated on one end of the middle of the three lines. The other end of the middle line is connected to ground.
The end of one of the two outer lines lying next to this line end in contact with ground is also in contact with ground, and its other end forms a balanced gate. One end of the other outer line is also in contact with ground, and the other end forms a second balanced gate. In this known balun, three line ends must be contacted with ground, for which purpose several contacts must be provided on a substrate carrying the line, which require relatively much space on the substrate. In order for the inputs of the balun not to be DC short-circuited relative to ground, capacitances must be inserted at all inputs for DC decoupling.
The underlying task of the invention is to provide a balun of the type just mentioned, in which DC decoupling is accomplished with the simplest possible means.
The mentioned task is solved with the features of Claim 1, in that, of the three planar lines connected to each other, one end of a first line running between a second and a third line serves as unbalanced gate and the other end of this first line is contacted with ground, and one end each of the other lines forms a balanced gate. DC decoupling is achieved by the fact that the ends of the secondhand third lines not serving as gates are capacitively coupled to each other.
As can be deduced from the subclaims, capacitive coupling of the line end can be accomplished very simply by the fact that the ends of the second and third lines are connected to line sections that run next to each other over a stipulated length, or by the fact that the ends of the second and third lines are connected to each other via one or more capacitors. It is expedient to connect a capacitor in series with the third line. This capacitor improves the balance between the balanced gates. It serves to balance the phase difference at 180xc2x0.